KR20170037660A - Determining a budget for lpd/fd transition frame encoding - Google Patents

Abstract

The present invention relates to a method for determining the distribution of bits for coding a transition frame. The method is implemented in an encoder / decoder for coding / decoding a digital signal. The transition frame is preceded by a predictive coded preceding frame. The coding of the transition frame includes transform coding and predictive coding of a single sub-frame of the transition frame. The method comprising the steps of: - allocating a bit rate for predictive coding of the transition sub-frame, the bit rate being equal to a minimum value between a first predetermined bit rate value and the bit rate for transform coding the transition frame 402; - determining (408, 408) a first number of bits allocated for predicting the transition subframe for the bit rate; And - computing (410) the number of possible bits for the transition frame coding and the second number of bits allocated for transcoding the transition frame, from the assigned first bit

Description

DETERMINING A BUDGET FOR LPD / FD TRANSITION FRAME ENCODING < RTI ID = 0.0 >

The present invention relates to the field of coding / decoding digital signals.

In order to efficiently code speech sounds at a low rate, CLEP (" Code Excited Linear Prediction ") technology is recommended. To effectively code music sounds, transcoding techniques are recommended.

The CELP type encoder is a predictive coder. Their goals include the use of fixed codebooks (white noise) to represent various elements: short-term linear predictions modeling vocal tracts, long-term predictions modeling vocal fold vibration during vocalization, and "innovations" (white noise), algebraic excitation, and so on).

A transcoder such as MPEG AAC, AAC-LD, AAC-ELD or ITU-T G.722.1 Annex C uses a critically sampled transform to compress the signal in the transform domain. The term " critically sampled transform "refers to a transform in which the number of coefficients in the transform domain in each analyzed frame is the same as the number of inner time domain samples.

One solution for effective coding of a signal including combined speech / music is to select the best technique over time between at least two coding modes: one is the CELP type and the other is the conversion type.

For example, codecs 3GPP AMR-WB + and MPEG USAC ("Unified Speech Audio Coding"). The target application of AMR-WB + and USAC is not a conversation, but a distribution and storage service without serious constraints on algorithm delay.

An early version of the USAC codec, referred to as RM0 (reference model 0), is described in M. Neuendorf et al., A Novel Scheme for Low Bit Rate Integrated Speech and Audio Coding of the 126th AES Convention, for Low Bitrate Unified Speech and Audio Coding) - Described in MPEG RM0 article. These RM0 codecs alternate between different coding modes.

Voice signal: LPD ("Linear Predictive Domain") mode containing two different modes derived from AMR-WB + coding:

- ACELP mode

(&Quot; Transform Coded Excitation ") called wLPT ("weighted linear predictive transform") using MDCT transformations (unlike AMR-WB + codecs) ) mode.

ㆍ Music signal: FD () which uses coding by MDCT ("Modified Discrete Cosine Transform") type of MPEG AAC ("Advanced Audio Coding") type using 1024 samples "Frequency Domain") mode.

The transition between LPD and FD mode in the USAC codec is crucial to guarantee sufficient quality without error in mode transition. Each mode (ACELP, TCX, FD) has a specific "signature" (in terms of artifacts) and the FD and LPD modes are of different types. The FD mode is based on transform coding in the signal domain and the LPD mode uses linear predictive coding in the perceptually weighted domain so that the filter memory is properly managed. Transition management between modes of the USAC RM0 codec is described in J. Lecomte et al., "Effective Cross-fade for LPC-based and Non-LPC-based Audio Coding Transitions," in the 126th AES Convention, Quot; Windows "article. As described in this article, the biggest difficulty is the transition from LPD to FD mode and vice versa. Here, only the case of transition from CELP to FD is discussed.

To fully understand how MDCT works, the principles of MDCT transform coding are recalled through common development practices.

In the encoder, the MDCT transform is typically divided into three stages, and the signal is subdivided into frames of M samples before MDCT coding:

• Assigning the weight of the signal to a window called "MDCT window" of 2M length.

Folding in the time domain to form a block of length M ("time domain aliasing").

ㆍ DCT transformation of length M.

The MDCT window is divided into four contiguous portions of the same length of M / 2, where the partitioned MDCT window is referred to as a "quarter ".

The signal is multiplied by an analysis window and then time domain aliasing is performed: The first quarter (window) is aliased on the second quarter (i.e., the time reversed and overlapped) , The fourth quota is aliased on the third quota.

More specifically, time-domain aliasing for the other quota of one quota is performed in the following manner: The first sample of the first quota is added to the last sample of the second quota (or subtracted from the last sample) The second sample of the first quota is added to (or added to) the first sample of the second quotient, such as added to (or subtracted from) the sample located immediately next to the last sample of the second quota Subtracted) is performed up to the last sample of the first quota.

Thus, two aliased quotas from four quarters are obtained, with each sample being the result of linear combination of two samples of the signal to be coded. This linear combination leads to time domain aliasing.

Thereafter, the two aliased quotas are jointly encoded after the DCT transform (Type IV). For the next frame, the third and fourth quotas of the preceding frame are shifted by half (i.e., 50% overlap) of the window to become the first and second quotas of the current frame. After aliasing, the second linear combination of sample pairs, such as in the preceding frame, is transmitted at different weights.

At the decoder, a decoded version of this wrapped signal is obtained after inverse DCT conversion. Two consecutive frames contain the results of two different overlaps of the same quotas. That is, it means that there is a result of two linear combinations with different known weights for each pair of samples: therefore, a system of equations can be solved to obtain a decoded version of the input signal, Aliasing can be eliminated by the use of two consecutive decoded frames.

The solution of the mentioned equation system is generally that of opening and multiplication of the synthesis window chosen judiciously, and between two consecutive decoded frames (without discontinuity due to quantization error) common It can be performed implicitly by adding and overlapping parts. In fact, this operation behaves like an overlap-add. When each sample of the window for the first or fourth quota is zero, it has an MDCT transformation without time-domain aliasing in that part of the window. In this case, a smooth transition due to the MDCT transformation is not provided and it must be performed by other methods, such as an external overlap addition.

It should be noted that, with respect to the time domain aliasing method of the block to be transformed, there is a modified implementation of the MDCT transform, in particular the definition of the DCT transform (for example, reversing the sign applied to the left and right aliased quota , And the second and third quotas may be aliased respectively to the first and fourth quotas). These transformations change the principles of MDCT analysis on windowing, time domain aliasing, and synthesis through transformation and sample block reduction by final windowing, aliasing, and overlap-add Do not.

In the case of the USAC RM0 coder described in a paper by Lecomte et al., The transition between a frame coded by ACELP coding and a frame coded by FD coding is performed in the following manner:

The transition window of the FD mode is used superimposed on the left side of 128 samples.

Time domain aliasing for this overlap region is canceled by introducing artificial time domain aliasing to the right of the reconstructed ACELP frame. The MDCT window used for the transition has a size of 2304 samples and the DCT transform operates on 1152 samples, but the FD mode frame is usually coded with a DCT transform of 1024 samples with a window size of 2048 samples. Thus, since the MDCT transform in normal FD mode can not be used directly in the transition window, the coder must incorporate a modified version of this transform that complicates the transition implementation for FD mode.

This delay is such that the coding delay is typically about 20-25 ms for voice coders of mobile applications (e.g., GSM EFR, 3GPP AMR and AMR-WB) and videoconferencing (e.g., UIT-T G.722.1 Annex C and G.719) is about 40 ms, which is not compatible with interactive applications. In addition, increasing the size of the DCT transform (2304 vs 2048) from time to time increases complexity at the moment of transition.

To overcome this drawback, International patent application WO2012 / 085451, incorporated herein by reference, provides a novel method of coding a transition frame. The transition frame is defined as the next transform coded current frame of the preceding frame coded by predictive coding. According to the novel method mentioned above, a part of the transition frame, for example a sub-frame of 5 ms for core CELP coding at 12.8 kHz and two additional CELPs of 4 ms each for core CELP coding at 12.7 kHz The frames are encoded by a more limited predictive coding than the prediction coding of the preceding frame.

The limited predictive coding consists of using stable parameters of the preceding frame encoded by predictive coding, such as, for example, coefficients of a linear prediction filter, and coding only a few minimum parameters for additional sub-frames in the transform frame .

Since the preceding frame is not encoded with transcoding, time-domain aliasing can not be undone in the first part of the frame. The above-cited patent application WO2012 / 085451 proposes modifying the first half of the MDCT window so that it does not have time-domain aliasing in the first normally-folded quota. Also, while changing the coefficients of the analysis / synthesis window, overlap-add (also referred to as "cross-fade") between the decoded CELP frame and the decoded MDCT frame ). ≪ / RTI > 4E of the above patent application, the dashed line (intersecting dots and dash) corresponds to the folding line (top of drawing) of MDCT encoding and the unfolding line (bottom of drawing) of MDCT decoding . The bold line in the above figure separates the frames of new samples into the encoder. The encoding of the new MDCT frame can begin when the thus defined frame of the new input sample is fully usable. Note that this bold line in the encoder corresponds to the incoming sample block for each frame rather than the current frame. The current frame is actually delayed by 5ms corresponding to the lookahead. At the bottom of the figure, the bold line separates the decoded frame at the decoder output.

In the encoder, the transition window is 0 until the folding point. Thus, the coefficient of the left side of the folded window is the same as the coefficient of the unfolded window. The portion between the folding point and the end of the CELP transition sub-frame (TR) corresponds to a sine (half) window. In the decoder, after being expanded, the same window is applied to the signal. The window count in the segment between the folding point and the beginning of the MDCT frame corresponds to a window of type sin ^ 2. In order to achieve overlap-add between the decoded CELP subframe and the signal from the MDCT, a window of the cos < 2 > type is applied to the overlap portion of the CELP subframe, it is sufficient to add the latter. This method provides a perfect reconstruction.

However, application WO2012 / 085451 provides for allocating a bit budget Btrans for coding a CELP subframe corresponding to a required budget for CELP coding a classic frame into a single subframe. The remaining budget for transform coding of the transition frame is insufficient and can lead to quality degradation at a low rate.

The present invention improves the situation.

To this end, a first aspect of the invention relates to a method for determining the distribution of bits for coding a transition frame. This method is implemented in a coder / decoder for coding / decoding a digital signal. Wherein the transition frame is preceded by a predictive coded preceding frame, and wherein coding the transition frame comprises transform coding and predictive coding of a single sub-frame of the transition frame. The method further comprises the following steps.

Allocating a bit rate for predictive coding of the transition subframe, the bit rate being equal to a minimum value between a first predetermined bit rate value and the bit rate for transform coding the transition frame;

- determining a first number of bits allocated for predicting the transition sub-frame for the bit rate; And

- calculating the number of possible bits for the transition frame coding and the number of second bits allocated for transcoding the transition frame from the assigned first bit.

In another embodiment, the coder / decoder includes a first core operation for predicting / decoding a signal frame at a first frequency and a second core operation for predicting / decoding a signal frame at a second frequency. The first predetermined bit rate value depends on the core selected from the first core operation and the second core operation for coding / decoding the predictive coded preceding frame.

In one embodiment, when the first core is selected to code / decode a predictive-coded preceding frame, the assigned bit rate is also associated with the bit rate of the transform-coded transition frame and at least one second predetermined bit rate Value and the second predetermined bit rate value is less than the first predetermined bit rate value.

In another embodiment, the digital signal is decomposed into at least one low band frequency and a high band frequency. In this situation, the calculated first number of bits is allocated to predictively code the transition sub-frame for the low-band frequency. A third predetermined number of bits is allocated to code the transition subframe for the highband frequency. In addition, the second number of bits allocated for transcoding the transition frame is determined from the third predetermined number of bits.

In one embodiment, the number of bits available for coding the transition frame is fixed.

In another embodiment, the second number of bits is equal to the number of fixed bits minus the first number of bits minus the third number of bits to code the transition frame. Thus, the final determination of the bit distribution in the transition frame is limited to subtracting the overall value, which simplifies the coding.

Alternatively, the second number of bits is equal to subtracting the first number of bits from the fixed number of bits, subtracting the third number of bits, and subtracting the first and second bits to code the transition frame. The first bit indicates whether low pass filter filtering is performed when determining the parameters of the predictive coding of the transition sub-frame, and the parameters are related to the tonal lead time. And the second bit indicates the frequency used by the coder / decoder for predictive coding / decoding the transition subframe.

A second aspect of the present invention is directed to a method of coding a digital signal in a coder capable of coding a signal frame in accordance with predictive coding or transform coding, the method comprising the steps of:

Coding the preceding frame (301) of digital signal samples according to predictive coding;

Coding the current frame of digital signal samples into a transition frame, wherein the step of coding the transition frame comprises a single sub-frame transform coding and a predictive coding of the transition frame, Wherein the steps comprise the following sub-steps:

Determining a distribution of bits according to one of the claims;

Coding the transition frame for the allocated second number of bits;

And predicting the transition sub-frame and the allocated first number of bits.

In one embodiment, predictive coding comprises generating determined predictive coding parameters for an assigned bit rate during distribution of bits in a transition frame. The use of such predictive parameters optimizes the ratio between the bit rate allocated for predictive coding and the remaining rate allocated for transform coding, thus optimizing the quality of the reconstructed signal. Indeed, at a constant quality, the number of bits attributed to the predictive parameter or other predictive parameter may vary at a non-linear rate relative to the bit rate allocated for predictive coding.

In another embodiment, the step of predictive coding comprises generating constrained predictive coding parameters associated with predicting the preceding frame by reusing at least one parameter for predictive coding of the preceding frame. Therefore, additional information is extracted from the preceding frame in decoding, and the transition is completed by decoding the transition sub-frame. This reduces the number of bits that must be reserved for predictive coding of the transition subframe.

Allocating the bit rate for the combination of reuse parameters from the preceding frame and the transform coding of the transition frame ensures low cost consistent transitions.

A third aspect of the present invention relates to a method for decoding a coded digital signal by predictive coding and transform coding, the method comprising the steps of:

Predicting and decoding a preceding frame of digital signal samples coded according to predictive coding;

Decoding the transition frame and coding a current frame of digital signal samples and coding the transition frame comprises transform coding and predictive coding of a single subframe of the transition frame, The sub-step comprising:

Determining a distribution of bits by a method according to the first aspect of the present invention;

Predicting and decoding the transition sub-frame with respect to the allocated number of bits;

- transforming and decoding the transition frame for the allocated second number of bits.

As described above, the method of determining the distribution of bits in the transition frame is directly reproducible in the decoder. Indeed, the distribution of the bits is determined only from the bit rate of the transform coded portion of the transition. Thus, no extra bits are needed to implement the step of determining the distribution of bits and bandwidth savings are achieved.

A fourth aspect of the present invention is directed to a computer program comprising instructions for implementing a method in accordance with aspects of the present invention as described above when the instructions are executed by a processor.

A fifth aspect of the present invention relates to an apparatus for determining a distribution of bits for coding a transition frame, the apparatus being implemented in a coder / decoder for coding / decoding a digital signal, Wherein the step of coding the transition frame comprises transcoding and predecoding a single subframe, the number of bits for coding the transition frame is fixed, and the device is next And a processor arranged to perform the operations:

Allocating a bit rate for predicting the transition subframe, the bit rate being equal to a minimum value between a bit rate for transform coding the transition frame and a first predetermined bit rate value;

Determining a first number of bits allocated for predicting the transition sub-frame for the bit rate;

- calculate a second number of bits allocated for transcoding the transition frame from the number of bits needed to code the coding parameters and the number of fixed bits for coding the transition frame.

A sixth aspect of the present invention is directed to a coder capable of coding digital signal frames in accordance with predictive coding or transform coding, the coder comprising:

An apparatus according to the fifth aspect of the present invention;

A prediction coder comprising a processor configured to perform the following operations, the operation comprising:

- code the preceding frame of digital signal samples according to prediction coding;

- coding the current frame of digital signal samples, and coding the transition frame, wherein said transform coding or predictive coding of the single sub-frame contained in the transition frame comprises the steps of: Arranged for predicting the assigned first number of transition subframes;

And a transform coder comprising a processor arranged to perform an operation of transform-coding the transition frame at the allocated second number of bits.

A seventh aspect of the present invention is directed to a decoder for a digital signal coded by predictive coding and transform coding, the decoder comprising:

A device according to the fifth aspect of the present invention;

A prediction decoder including a processor configured to perform the following operations:

Predicting and decoding the preceding frame of the coded digital signal samples according to predictive coding;

- predicting and decoding a single subframe included in a transition frame, coding a current frame of digital signal samples, coding the transition frame, predictively coding and transform coding the subframe, Arranging to perform an operation of predicting and decoding the transition sub-frame,

And a transposition decoder including a processor arranged to perform an operation of performing a transition transform coding of the transition frame with respect to the allocated second number of bits.

The present invention is advantageously applied to coding / decoding sounds that may include mixed speech and audio mixed together.

According to the present invention, the predictive coding bit rate is suppressed by the maximum value. The number of bits allocated to predictive coding depends on these bit rates. As the bit rate is weaker, the minimum remaining budget for transition frame transform coding is guaranteed, since the number of bits allocated for coding is less.

In addition, the consistency of the generated signal is improved, simplifying the coding (channel coding) and the subsequent steps of processing the received frame at the decoder.

Also, a minimum bit rate is guaranteed to prevent too large rate differences between different coded frames.

It is also possible to efficiently code the entire frequency spectrum of the input signal without sacrificing the quality of the recovered signal during decoding.

In addition, the number of available bits for coding the transition frame is fixed. This reduces the complexity of the coding step.

It also permits more flexible coding by indicating the frequency used by the coder / decoder for predictive coding / decoding.

Also, determining the bit distribution is reproducible in the decoder and prevents explicit transmission of information about this distribution.

In addition, such coding ensures a balanced distribution between predictive coding and transform coding within this transition frame.

It also optimizes the quality of the reconstructed signal. Indeed, at a constant quality, the number of bits attributed to the predictive parameter or other predictive parameter may vary at a non-linear rate relative to the bit rate allocated for predictive coding.

Further, additional information is extracted from the preceding frame in decoding, and the decoding is completed by decoding the transition sub-frame. This reduces the number of bits that must be reserved for predictive coding of the transition subframe.

Assigning a combination of reuse parameters from the preceding frame and a bit rate for transcoding the transition frame ensures a low cost consistent transition.

As described above, the method of determining the distribution of bits in the transition frame is directly reproducible in the decoder. Indeed, the distribution of the bits is determined only from the bit rate of the transform coded portion of the transition. Thus, no extra bits are needed to implement the step of determining the distribution of bits and bandwidth savings are achieved.

Other features and advantages of the invention will appear when the following detailed description and accompanying drawings are considered.
1 illustrates an audio coder according to an embodiment of the present invention.
FIG. 2 is a diagram illustrating steps of a coding method implemented by the audio coder of FIG. 1 according to an embodiment of the present invention.
Figure 3 illustrates the transition between a CELP frame and an MDCT frame in accordance with an embodiment of the present invention.
4 is a diagram illustrating steps of a method for determining a distribution of bits for coding a transition frame according to an embodiment of the present invention.
5 illustrates an audio decoder according to an embodiment of the present invention.
FIG. 6 is a diagram illustrating steps of a decoding method implemented by the audio decoder of FIG. 5 according to an embodiment of the present invention.
Figure 7 illustrates an apparatus for determining the distribution of bits in a transition frame according to an embodiment of the invention.

Figure 1 illustrates an audio coder 100 in accordance with an embodiment of the present invention.

FIG. 2 is a diagram illustrating the steps of a coding method implemented by the audio coder 100 of FIG. 1, in accordance with an embodiment of the invention.

The coder 100 includes a receiving unit 101 that receives samples at a given frequency fs (e.g., 8, 16, 32 or 48 kHz) in step 201 and is decomposed into sub-frames, for example 20 ms.

Upon receiving the current frame, the pre-processing unit 102 may select, in step 202, a coding mode best suited to code the current frame between the at least one LPD mode and the FD mode. In the following description, for illustrative purposes, it is contemplated that MDCT coding is used for the FD mode and CELP coding is used for the LPD mode. There are no restrictions on the coding techniques used for each of the LPD and FD modes. Thus, modes other than CELP and MDCT modes may be used, for example CELP coding may be replaced with other types of predictive coding, and MDCT transforms may be replaced with other types of transforms.

For example, assume that the frame type is explicitly transmitted via block 206 with fixed length coding indicating a mode selected from a predetermined list. In a variant of the invention, this coding for the selected mode in each frame may be variable length. The CELP coding type (12.8 or 16 kHz) may be explicitly transmitted via bits to facilitate decoding of the transition frame.

In step 203, it is verified in step 202 whether CELP decoding is selected. When the LPD mode is selected, the signal frame is transmitted to the CELP coder 103 in step 204 to code the CELP frame. The CELP coder may use, for example, two "cores" operating at two separate internal sampling frequencies fixed at 12.8 kHz and 16 kHz, and the input signal (at frequency fs) at an internal frequency of 12.8 or 16 kHz The use of sampling is required. Such re-sampling may be implemented in the pre-processing block 102 or the resampling unit of the CELP coder 103. [ The frame is typically predictively coded by the CELP coder 103 by subtracting the CELP parameter according to the signal classification. CELP parameters generally include LPC coefficients, fixed and adaptive gain vectors, adaptive dictionary vectors, and fixed dictionary vectors. This list may be modified based on the signal category of the frame, as in UIT-T G.718 coding. The calculated parameters may thus be quantified, multiplexed and transmitted to the decoder by the transmission unit 108 in step 206. [ If the frame following the current frame is an MDCT transition frame, CELP coding parameters such as LPC coefficients, fixed and adaptive gain vectors, adaptive dictionary vectors, fixed dictionary vectors, and CELP decoder states may further be stored in memory 107 in step 205 have.

As described below, the band extension may be performed with coding related to the high band if the current frame is a CELP type.

If the MDCT coding is selected by the unit 102 in step 203, it is confirmed in step 207 that the frame preceding the current frame is MDCT transform coded. If the frame preceding the current frame has been MDCT transform coded, the current frame is sent directly to the MDCT coder 105 for MDCT transform coding the current frame in step 208. [ The MDCT coder can code frames that include a 28.75 ms non-resampled signal, including a 20 ms frame and a 8.75 ms lookahead. The size of the MDCT window is unlimited. Also, the delay corresponding to the CELP coder delay due to resampling of the input signal is applied to the frame coded by the MDCT coder in such a way that the MDCT and CELP frames are synchronized. This delay in the coder may be 0.9375 ms depending on the resampling type prior to CELP coding. The MDCT transform coded frame is sent to the decoder in step 206. [

When MDCT coding is selected by the unit 102 and the frame preceding the current frame is predictively coded, the current frame is transferred to the transition unit 104 as a transition frame. As described below, the MDCT transition frame includes additional CELP subframes.

Transition unit 104 may implement the following steps:

- In step 209, estimating the budget of the bits necessary for coding the transition CELP sub-frame to define the budget available for the MDCT coding the current frame. As described below, the budget may vary depending on the current frame rate. The budget can also be evaluated according to the CELP core used. In order to preserve a sufficient bit budget not to degrade the quality of the MDCT coding, the present invention may provide for limiting the coding rate for CELP subframes. To this end, it includes an apparatus for determining a bit distribution within a transition frame, such as apparatus 700 of Figure 7;

- In step 210, modifying the MDCT window used in the coder according to FIG. 3 to be described later;

- In step 207, the MDCT memory can be ignored in MDCT decoding in the same way as the step of zeroing the MDCT transform memory because the preceding frame is a CELP frame.

In one embodiment, at least one of these steps is performed by a transition frame coding unit 106 described below.

The transition MDCT frame is coded by the MDCT coder 105 in step 212 and coded based on the budget of the bits allocated in step 209, as described below. Further, the additional CELP subframe may be coded by the CELP coder 103, as described below with reference to FIG. 3, in step 213, and may be determined according to the bit budget allocated in step 209. [ CELP coding may be performed before or after MDCT coding.

Figure 3 shows the transition between CELP and MDCT frames before coding, before coding of the coder and before decoding of the decoder.

The frame for the code 301 is received at the coder 100 and coded by the CELP coder 103. The current frame 302 is received by the input of the coder 100 to be MDCT transform coded. Therefore, this is a transition frame. Also, the next frame 303 received by the input of the coder is MDCT transform coded. According to the present invention, the subsequent frame 303 may be coded by CELP coding, and there is no restriction on the coding used for the next frame 303. [

The asymmetric MDCT window 304 may be used to code the current frame. This window 304 has a rising edge 307 of 14.375 ms, a level with a gain of 1 of 11.25 ms, a falling edge 309 of 8.75 ms corresponding to the lookahead, and a null portion of 5.265 ms Gt; 310 < / RTI > Adding the null portion 310 reduces the seer and thus the corresponding delay. In one embodiment, the form of the MDCT analysis window for MDCT coding is modified, for example, to further reduce the seer or to use a symmetrical window with the examples given in patent application WO2012 / 085451.

Dotted line 312 represents the media of the MDCT window 304. On either side of the line 312, the 10 ms quota of the MDCT window 212 is aliased as described in the introduction. Solid line 311 represents the aliased area between the first and second quotas of the MDCT window 304. [ The MDCT window of the next frame 303 is referenced 306 and shows the overlap-add region with the MDCT window 304 corresponding to the falling edge 309 of the MDCT window 304. [

The MDCT window 305 theoretically represents the window to be applied to the previous window if it is MDCT transform coded. However, if the preceding frame 301 is coded by the CELP coder 103, the window is null in the first quota to allow the decoder to open the first part of the MDCT transform coded frame Since the second portion of the MDCT frame of FIG.

For this purpose, the MDCT window 304 is modified by the MDCT window 313 with the first quota at zero, allowing time-domain aliasing in the first part of the MDCT frame at the decoder.

In the decoder, the analysis windows 304, 305, 306, and 313 correspond to the synthesis windows 324, 325, 326, and 327, respectively. The synthesis window is therefore time-reversed with respect to the corresponding analysis window. In a variant of the invention, the analysis and synthesis window may be of the same or sinusoidal type or otherwise.

The first frame 320 of new samples coded by CELP coding is received at the decoder. It corresponds to the coded version of this CELP frame 301. Here, it is recalled that the decoded frame is shifted by 8.75 ms relative to frame 320.

A coded version of the transition frame 302 is received (see 321 and 222 which form the complete frame). A gap is created between the end of the CELP frame 320 and the beginning of the rising edge of the synthesis window 327 (corresponding to the aliasing line). In the specific example shown here, 1/4 of the MDCT window is 10 ms and the null portion of the composite window MDCT 324 covering this CELP frame 220 is 5.625 ms (part 310 of the MDCT analysis window 204) , The gap is 4.275 ms. In addition, a delay between the start of this CELP frame 320 and the start of the MDCT window 327 is necessary to ensure a satisfactory overlap-add length along with the start of the non-null portion of the MDCT window 327 Lt; / RTI > In the following example, for illustrative purposes, a satisfactory overlap-add length of 1.875 ms is considered, and the delay described above (corresponding to the missing signal length), as represented by reference numeral 321 in Figure 2, ms.

It should be noted that the signal frames shown in FIG. 3 may include signals at different sampling frequencies of 12.8 or 16 kHz in the case of CELP coding / decoding and may include fs in the case of MDCT coding / decoding. However, at the decoder, after a time shift of the resampling and MDCT synthesis of the CELP synthesis, the frames remain synchronized and the representation of Fig. 3 remains accurate.

As described above, application WO2012 / 085451 codes additional CELP subframes of 5 ms at the beginning of the MDCT transition frame for 12.8 kHz CELP coding and 4 ms at the beginning of the MDCT transition frame for 16 kHz CELP coding Coding two additional CELP frames.

At 12.8 kHz, the 6.25 ms delay is not satisfied and the overlap-addition is affected: only a nesting-add of 0.625 ms is not enough at the decoder.

In the case of 16 kHz, two additional CELP subframes are coded at the beginning of the transition frame, which leaves little budget for coding the transition MDCT frame and can result in significant degradation at low rates.

To overcome this drawback, the present invention proposes the coding of one additional CELP subframe at 12.8 or 16 kHz by the CELP coder 103. To generate the above-mentioned 6.25 ms long missing signal, additional samples are generated at the decoder as will be described in detail below.

To code the transitional CELP subframe, the unit 106 may reuse at least one CELP parameter of the preceding CELP frame. For example, unit 106 may convert energy from previous frame innovations (stored in memory 107 as previously described) as well as the linear prediction coefficients A (z) of the previous CELP subframe into an adaptive dictionary vector, Gain, fixed gain, and the fixed pre-vector of the transition CELP sub-frame. Thus, the additional CELP subframe may be coded with the same core (12.8 kHz or 16 kHz) as the preceding CELP frame.

Transition frame coding unit 106 ensures transition frame coding according to the present invention. The present invention may further provide for insertion by the unit 106 in the bit stream of additional bits indicating that the coded frame 322 is a transition frame, but in general cases, such a transition frame indication may be made by taking the extra bits Can be transmitted to the global indication of the current frame coding mode without any indication.

Also, since the present invention does not require that the sample frequency of the synthesized signal of the decoder need to be equal to the CELP core frequency, the unit 116 may perform steps 204 and 214 (so-called "band extension" Method) can code a high-band signal at a fixed budget.

For this purpose, the coding unit of the transition frame 106 may implement the following steps:

Filtering the CELP leading frame and the CELP subframe of the transition frame by a high pass filter to preserve a high portion of the spectrum (above the frequency corresponding to the CELP core used, i.e. above 6.4 or 8 kHz). This filtering may be implemented by a filter having a finite impulse response (FIR) from the CELP coder 103;

- searching for a correlation between the filtered portion of the original transitional CELP sub-frame and the filtered preceding CELP frame to estimate the delay parameter and gain (prediction by applying a signal and a delay corresponding to the filtered sub- ≪ / RTI >

For example, using a scalar quantification to code a delay parameter and the gain described above (e.g., the delay can be coded to 6 bits or more, and the gain can be coded to 6 bits or more ).

The above-described step 209 is described in more detail with reference to FIG. 4, which shows the steps of a method for determining a bit distribution for transform coding according to an embodiment of the present invention. The above-described method is performed in the same manner in the coder and decoder, but is shown on the coder side only for the purpose of explanation.

In step 400, the total rate (bits / s) indicated by core_brate, which can be assigned to the current frame coding, is fixed equal to the output rate of the MDCT coder. In this example, the duration of the frame considered as 20 ms, the number of frames per second is 50, and the total budget in bits is equal to core_brate / 50. The total budget may be fixed for a fixed rate coder, or may be variable for a variable rate coder. In the following, the num_bits variable is used and initialized to the core_brate / 50 value.

In step 401, the transition unit 104 determines the CELP core from at least two CELP cores used to code this preceding CELP frame. In the following example, two CELP cores are considered and operate at frequencies of 12.8 kHz and 16 kHz, respectively. Alternatively, a single CELP core is implemented upon coding and / or decoding.

If the CELP core used in the preceding CELP frame has a frequency of 12.8 kHz, the method includes allocating (402) a bit rate labeled cbrate for CELP coding the transition subframe, Is equal to the minimum value between the bit rate for the MDCT coding of the transition frame and the first predetermined bit rate value. The first predetermined value may be fixed, for example, to 24.4 kbit / s, which ensures a satisfactory bit budget for transmission coding.

Thus, cbrate = min (core_bitrate, 24400). This limitation is equivalent to inhibiting the operation of limited CELP coding, limited to additional subframes with coded CELP parameters as coded at most by 24.40 kbit / s CELP coding.

In an optional step 403, the allocated bit rate is compared to the 11.60 kbit / s CELP bit rate. If the allocated bit rate is higher, the bits for coding the bit indications for low pass filtering an adaptive dictionary can be reserved (e.g., higher than or equal to 12.65 kbit / s for the AMR- WB coding). The num_bits variable is updated.

num_bits: = num_bits - 1

In step 404, the first number of bits labeled budg1 is allocated to predictively code additional CELP subframes. The first number of bits (budg1) represents the number of bits representing the CELP parameter used to code the CELP subframe. As described above, the coding of the CELP subframe uses a limited number of CELP parameters, and some of the parameters used to code the preceding CELP frame can be advantageously re-used.

For example, only excitation can be modeled to code additional CELP subframes, so the bits are reserved only for fixed dictionary vectors, adaptive dictionary vectors, and gain vectors. The number of bits attributed to each of these parameters is deduced from the bit rate assigned in step 402 to code this additional CELP subframe. For example, the bit distribution of the AMR-WB encoding algorithm for Table 1 / G722.2-20 ms frames, started in the July 2003 version of G.722.2 of ITU-T, is shown in the CELP parameter ≪ / RTI >

In the previous example, if subframe coding is restricted, budg1 corresponds to the sum of the bits due to each of the adaptive dictionary, the fixed dictionary and the gain vector. For example, when the allocated bit rate is 19.85 kbit / s, 9 bits are allocated to the fixed dictionary (tone lead time) and 7 bits are allocated to the gain vector (directory gain) . In this case, budg1 is equal to 88 bits.

The num_bits variable can be updated:

num_bits: = num_bits - budg1

The present invention may also provide for considering frame categories in the allocation of bits for CELP parameters. For example, sections 6.8 and 8.1 of the G.718 norm, the June 2008 version of ITU-T, describe categories, modes such as non-voice mode (UC), voice mode (VC) And a budget allocated to each CELP parameter depending on the normal mode (GC), or the assigned bit rate (layer 1 or layer 2 corresponding to 8 kbit / s and 8 + 4 kbit / s speed, respectively). Coder G.718 is a hierarchical coder, but it is possible to combine CELP coding principles using G 718 categorization with multi-rate allocation of AMR-WB.

If it is determined in step 401 that the CELP core used for the preceding CELP frame has a frequency of 16 kHz, the method includes step 405 of assigning a bit rate (labeled cbrate) for CELP coding the transition sub-frame , The bit rate being equal to a minimum value between a bit rate for the MGCT coding the transition frame and a first predetermined value of the bit rate. For a 16 kHz core, the first predetermined value can be fixed, for example, to 22.6 kbit / s, which allows to ensure a satisfactory bit budget for transform coding. Thus, the first predetermined value depends on the CELP core used in the code of the preceding CELP frame. Also, in order to code a 16 kHz core, a threshold value can be applied when assigning a bit rate to the CELP coding. Thus, the assigned bit rate is equal to the maximum value between the bit rate for the transcoded transition frame and the at least one predetermined second bit rate value, and the second value is lower than the first value. The second predetermined value of the trade may be, for example, 14.8 kbit / s. Thus, if the bit rate for the transform coded transition frame is lower than 14.8 kbit / s, the bit rate allocated for the CELP coding the transition sub-frame may be 14.8 kbit / s.

In one complementary embodiment, if the bit rate for the transform coded transition frame is less than 8 kbit / s, the allocated rate may be 8 kbit / s.

Thus, according to this complementary embodiment, the following algorithm is obtained:

If core_bitrate ≤ 8000

cbrate = 8000

Otherwise if core_bitrate ≤ 14800

othercbrate = 14800

Otherwise

cbrate = min (core_bitrate, 22600)

End if

In an optional step 407, the allocated bit rate is compared to a CELP bit rate of 11.60 kbit / s. If the assigned bit rate is higher, the bit may be reserved to code the low-pass filtering bit representation of the adaptive dictionary. The num_bits variable is updated.

num_bits: = num_bits - 1

In step 408, in the same manner as step 404, a first number of bits budg1 is allocated to predictively code additional CELP subframes, and budg1 depends on the bit rate allocated for CELP coding the transition subframe.

In a step 410 common to coding at various core frequencies, a second number labeled budg2 is assigned for transform coding of the transition frame and is calculated from the first number of bits, budg1, which is the total number of bits of the transition frame. With respect to the above calculation, budg2 is the same as the num_bits variable. In general, the mode of the transition current frame is assumed to belong to the MDCT coding budget, so this information is not explicitly considered.

If the audio signal is decomposed into at least one low band frequency and one high band frequency, the preceding steps can be implemented to code the low band frequency of the transition sub-frame. In an optional step 409 prior to step 410 common to coding at different core frequencies, the method includes assigning a third predetermined number of bits labeled as budg3 to code the frequency high band of the transition subframe . ≪ / RTI > In this case, the second number of bits (budg2) is calculated from the first number of bits (budg1) and the third number of bits (budg3).

As previously described, coding the high band frequency (or extending the band) of the transition subframe may be based on the correlation between the preceding frame and the transition subframe of the audio signal. For example, coding high band frequencies can be broken down into two stages.

In the first step, the preceding frame and the current frame of the audio signal are filtered by a high pass filter to maintain only the upper part of the spectrum. The upper part of the spectrum may correspond to a frequency higher than the frequency of the CELP core used. For example, if the CELP core used is a 12.8 kHz CELP core, the high band corresponds to a filtered audio signal with a frequency lower than 12.8 kHz. This filtering can be implemented by a FIR filter.

In the second step, a correlation search is performed between the preceding frame and the filtered portions of the current frame. This correlation search allows estimating the delay parameter and then estimating the gain. The gain corresponds to the amplitude ratio between the filtered portion of the current frame and the predicted signal by applying a delay.

For example, 6 bits may be allocated for gain and 6 bits for delay. And the third bit number (budg3) becomes 12.

The num_bits variable can be updated.

num_bits: = num_bits - budg3

Thus, the second number of bits (budg2) is equal to the updated variable num_bits.

FIG. 5 illustrates an audio decoder 500 in accordance with an embodiment of the present invention, FIG. 6 illustrates a diagram illustrating the steps of a decoding method according to an embodiment of the present invention implemented in the audio decoder 500 of FIG. to be.

The decoder 500 includes a receiving unit 501 for receiving the coded digital signal (or bit flow) generated from the coder 500 of FIG. If the current frame is a CELP frame, an MDCT frame, or a transition frame, the bit flow is submitted to the categorization unit 502, which can be determined in step 602. [ To this end, the categorization unit 502 may subtract the bit flow from the bit-flow information indicating whether the current frame is a transition frame and from which information the CELP core uses to decode the CELP frame or the transition CELP sub-frame .

In step 603, it is confirmed that the current frame is a transition frame.

If the current frame is not a transition frame, it is confirmed in step 604 that the current frame is a CELP frame. If so, the frame is transmitted to the CELP decoder 504, which can decode the CELP frame in step 605, with the core frequency indicated by the categorization unit 502. [ After decoding the CELP frame, the CELP decoder 504 stores internal states, such as parameters, such as the linear prediction filter coefficients A (z), and the predicted energy in the case where the next frame is a transition frame, ).

As an output from the CELP decoder 504, the signal may be resampled by the resampling unit 505 to the output frequency of the decoder 500 in step 607. [ In one embodiment of the invention, the resampling unit comprises a FIR filter and the resampling introduces a delay of, for example, 1.25 ms. In one embodiment, the post-processing may be applied to CELP decoding before or after resampling.

As described above, in one embodiment, band extension may be performed with the decoding associated with the high band when the current frame is the CELP type by the management unit 5051 of band extension in steps 6071 and 6151. The highband is then combined with CELP coding, potentially allowing additional delay to be applied to CELP synthesis in the low band.

A signal that is potentially post-processed and resampled before or after being decoded and resampled by the CELP decoder is transmitted to the output interface 510 of the decoder in step 608. [

The decoder 500 further includes an MDCT decoder 507. If it is determined in step 604 that the current frame is an MDCT frame, the MDCT decoder 507 may decode the MDCT frame in a classical manner in step 609. [ In step 610, the delay corresponding to the delay required for the resampling application of the signal generated from the CELP decoder 504 is applied to the decoder output by the delay unit 508 to synchronize the MDCT synthesis with the CELP synthesis. The signal decoded and delayed by the MDCT is transmitted to the output interface 510 of the decoder in step 608. [

If the current frame is determined to be a transition frame after step 603, the apparatus for determining the distribution of bits 503 determines in step 611 the first bit number (budg1) allocated for CELP coding of the transition frame and the transformed coding Lt; / RTI > (budg3) that is allocated for the first bit. The device 503 may correspond to the device 700 described in detail with reference to FIG.

The MDCT decoder 507 uses the third bit number (budg3) calculated by the decision unit 503 to adjust the rate required for decoding the transition frame. The MDCT decoder 507 further zeroizes the memory of the MDCT transform and decodes the transition frame in step 612. [ The signal generated from the MDCT decoder is delayed by the delay unit 508 in step 613.

At the same time, in step 614, the CELP decoder 504 decodes the transition CELP subframe based on the first number of bits budg1. For this purpose, the CELP decoder 504 decodes a CELP parameter that may depend on the current frame category and includes, for example, a pitch value from the adaptive dictionary, a fixed and gain dictionary of the CELP subframe, Filter coefficients are used. In addition, the CELP decoder 504 updates the CELP decoding state. These states may include the predictive energy of innovation from a preceding CELP frame to generate a signal sub-frame over 4 ms or 5 ms, typically depending on whether a 12.8 kHz or 16 kHz CELP core is used (If the transitional CELP subframe is limited coding).

As described above, application WO2012 / 085451 provides a further coding of two additional sub-frames of 4 ms for a 5 ms sub-frame for a CELP core of 12.8 kHz and a CELP core of 16 kHz.

As described with reference to FIG. 3, for 12.8 kHz, the 6.25 ms delay is not satisfied and the superposition-addition is affected: the decoder only has a superposition-add of 0.625 ms, which is insufficient.

At 16 kHz, for an additional CELP sub-frame, it is coded at the beginning of the transition frame, which leaves little budget for coding the transition MDCT frame, and is a "full-rate" MDCT It may cause deterioration in quality of coding.

Thus, the solution of international application WO2012 / 085451 is unsatisfactory.

An independent aspect of the present invention partially generates a second sub-frame by reusing, from one additional transition CELP sub-frame, the coding parameters used to encode the transition CELP sub-frame. Thus, the delay guarantees sufficient overlap addition and is filled without affecting the MDCT coding rate of the transition frame.

For this purpose, the present invention also aims at a method for decoding a coded digital signal P in a decoder 500 that is capable of decoding a signal frame according to predictive decoding or in accordance with transform decoding. The method may comprise the following steps:

- receiving, in step 501, a first set of predictive coding parameters for coding a first digital signal frame;

- predicting the first frame based on the first set of predictive coding parameters in step 605;

Receiving a second set of parameters for predicting a first transition subframe of the transform coded transition frame in step 501 for the new frame;

- decoding the first transition sub-frame based on the second set of predictive coding parameters in step 614,

- generating, in step 614, samples from the second transition sub-frame from the at least one predictive coding parameter of the second set.

The present invention also contemplates a computer program that includes the decoder 500 for implementing a method for decoding P, as well as the instructions for performing the method of decoding P when the instructions are executed by a processor.

The CELP parameters reused to generate the second subframe may be a gain vector, an adaptive dictionary vector, and a fixed dictionary vector.

According to one embodiment of the method for decoding P, the minimum overlap value can be predefined for transform decoding, and the number of samples generated from the second subframe is determined based on the minimum overlap value. This last sub-frame extends the CELP synthesis by repeating the pitch prediction with the same pitch delay and the same adaptive pre-gain as in the first sub-frame and uses the same LPC coefficients and de-emphasis or de-accentuation ) By performing the synthetic LPC filtering.

The second CELP subframe preserves a 1.25 ms signal for a 12.8 kHz CELP core and can be truncated for a 2.25 ms signal for a 16 kHz CELP core. Thus, the first CELP subframe is filled with a 6.25 ms additional signal to fill the gap and ensure a satisfactory overlap-add (minimum overlap value, e.g., 1.875 ms) with the MDCT transition frame. In one embodiment, the additional CELP subframe has an extended length of 6.25 ms for the 12.8 and 16 kHz CELP cores, which is particularly suitable for fixed dictionaries as "normal" CELP coding with such extended subframe length Means to modify

In addition to the previous embodiment of the method of decoding P, method P may further comprise resampling 615 performed by a finite impulse response filter. As previously described, the FIR filter may be integrated into resampling unit 505. [ Resampling uses the FIR filter memory from the preceding CELP frame and processing leads to an extra delay of 1.25 ms in this example.

The method P may further comprise adding an additional signal obtained from the samples stored in the finite impulse response filter memory to fill the delay introduced by the resampling step. Thus, in addition to the 6.25 ms additional signal previously generated, a 1.25 ms signal is generated by the decoder 500 and these samples advantageously allow the delay introduced by resampling of the 6.25 ms additional signal .

For this purpose, the FIR filter memory of resampling unit 505 may be stored for each frame after CELP decoding. The number of samples in this memory corresponds to 1.25 ms at the considered CELP core frequency (12.8 or 16 kHz)

According to a complementary embodiment of method P, resampling the stored samples is performed by an interpolation method which introduces a second delay shorter than the first delay from the finite impulse response filter, which can be considered null . Thus, the 1.25 ms signal generated from the FIR filter memory is resampled according to a method which means minimum delay. For example, resampling a 1.25 ms signal generated by an FIR filter memory can be performed by cubic interpolation, which means a delay from only two samples, and the delay from the FIR filter ≪ / RTI > Thus, two additional samples are needed to resample the 1.25 ms signal mentioned above. These two additional samples can be obtained by repeating the last value of the resampling memory of the FIR filter.

The decoder may further decode the high frequency portion from the 6.25 ms CELP signal obtained from the first and second transition frames. To this end, the CELP decoder 504 may use an adaptive gain and a fixed dictionary vector from the last subframe of the preceding CELP frame.

Decoder 500 can ensure overlay addition between the decoded signal and the decoded signal of the resampled CELP transition subframe, the sample resampled by cubic interpolation and the transition frame generated from MDCT decoder 507 in step 616 And a superimposing addition unit 509. [

For this purpose, the unit 509 applies the synthetic modified window 327 of FIG. Thus, prior to the MDCT aliasing point for both quotas, the samples are zeroed. After the aliasing point described above, the windowed samples are divided by the unmodified window 324 of FIG. 3 and multiplied by a sinus-type window associated with the window applied to the encoder, ^ 2. Samples generated by CELP and 0-delayed resampling (eg, cubic interpolation) at the portion associated with the overlap addition are weighted by the cos ^ 2 window.

The thus obtained transition frame is transmitted to the output interface 510 of the decoder in step 608. [

FIG. 7 shows an example of an apparatus 700 for determining a bit distribution for a transition frame.

The apparatus includes a processor 703 for storing instructions that may implement a random access memory 704 and a method for determining the bit distribution for the transition frame described above. The apparatus also includes a mass memory (705) for storing data intended to be stored after applying the method. Device 700 also includes an input interface 701 and an output interface 706 for receiving digital signal frames and for emitting the details of the budget allocated to each of these different frames.

The device 700 may further include a digital signal processor (DSP) 702. The DSP 702 may receive digital signal frames for forming, demodulating, and amplifying these frames in a known manner.

The present invention is not limited to the embodiments described above for illustrative purposes only; It extends to other variants.

Thus, an embodiment has been described in which the compression or decompression apparatus is an entity as a whole. Of course, the device may be embedded in any type of more important device, such as, for example, a digital camera, a photographic camera, a mobile phone, a computer, a movie projector,

In addition, embodiments have been described which suggest specific designs of compression, decompression and comparison devices. These designs are provided for illustrative purposes only. Thus, the arrangement of components and the different distributions of tasks assigned to each component can also be considered. For example, tasks performed by a digital signal processor (DSP) may be performed by a classic processor.

100: Audio coder
101: Receiving unit
102: preprocessing unit
103: CELP coder
104: transition unit
105: MDCT coder
106: Frame coding unit
107: Memory
108: Transmission unit
500: Audio decoder
501: receiving unit
502: Categorization unit
504: CELP decoder
505: resampling unit
507: MDCT decoder
508: Delay unit
509: Overlap addition unit
510: Output interface

Claims (15)

CLAIMS What is claimed is: 1. A method for determining a distribution of bits for coding a transition frame (321, 322), implemented in a coder / decoder (100; 500) for coding /
The transition frame precedes the predictive coded preceding frame 320,
Wherein the coding of the transition frame comprises transform coding and predictive coding of a single sub-frame of the transition frame,
- allocating a bit rate for predictive coding of the transition subframe, the bit rate being equal to a minimum value between a first predetermined bit rate value and the bit rate for transform coding the transition frame (402; 405 );
- determining (408, 408) a first number of bits allocated for predicting the transition subframe for the bit rate; And
- calculating (410) the number of possible bits for the transition frame coding and the second number of bits allocated for transcoding the transition frame from the assigned first bit. The method according to claim 1,
The coder / decoder (100; 500)
A first core operation for predictive coding / decoding a signal frame at a first frequency and a second core operation for predictive coding / decoding a signal frame at a second frequency,
Wherein a second predetermined bit rate value is dependent on a core selected from the first core operation and the second core operation for coding / decoding the predictive coded preceding frame (320). 3. The method of claim 2,
When the first core is selected to code / decode the predictive coded preceding frame 320, the assigned bit rate is compared to the bit rate of the transform coded transition frame 322 and the at least one second Wherein the second predetermined bit rate value is equal to a maximum value between the determined bit rate values, and wherein the second predetermined bit rate value is less than the first predetermined bit rate value. 12. A compound according to any one of the preceding claims,
The digital signal is decomposed into at least one low band frequency and a high band frequency,
The calculated first number of bits is allocated for predictive coding of the transition sub-frame 321 for the low-band frequency,
A third predetermined number of bits is allocated for coding of the transition sub-frame for the high-band frequency,
Wherein the second number of bits allocated for transcoding of the transition frame (322) is determined from the third predetermined number of bits. 5. The method of claim 4,
Characterized in that the number of available bits for coding the transition frames (321, 322) is fixed. 6. The method of claim 5,
Wherein the second number of bits is equal to a value obtained by subtracting the first number of bits from a fixed number of bits to code the transition frame (321; 322) and subtracting the third number of bits. 6. The method of claim 5,
The second number of bits is calculated by subtracting the first number of bits from a fixed number of bits to code the transition frame 321 (322), subtracting the third number of bits, subtracting the first and second bits, The same,
Wherein the first bit indicates whether low pass filter filtering is performed when determining the parameters of the predictive coding of the transition subframe, the parameters being related to a tonal lead time,
And the second bit indicates a frequency used by the coder / decoder for predictive coding / decoding the transition subframe. 1. A method of coding a digital signal in a coder (100) capable of coding a signal frame in accordance with predictive coding or in accordance with transform coding,
Coding the preceding frame (301) of digital signal samples in accordance with predictive coding; And
Coding the current frame (302) of digital signal samples into a transition frame (321, 322), wherein coding the transition frame (321, 332) comprises transforming a single sub- Coding and predictive coding, the step of coding the current frame (302) comprises the following sub-steps:
- determining (209) the distribution of bits according to any one of the preceding claims;
- transform coding (212) the transition frame (322) for a second number of bits allocated; And
- predicting the transition sub-frame (321) and the first number of bits allocated (213). 9. The method of claim 8,
Wherein the predictive coding step comprises generating determined predictive coding parameters for the assigned bit rate. 10. A method according to any one of claims 8 and 9,
Wherein the predictive coding step includes reusing at least one parameter for predictive coding of the preceding frame to generate limited predictive coding parameters associated with predictive coding of the preceding frame. A method of decoding a coded digital signal, wherein the decoder (500) implemented in accordance with the method is capable of decoding signal frames in accordance with predictive coding or transform decoding,
Predicting (605) a preceding frame of digital signal samples coded according to predictive coding; And
Decoding the transition frames 321 and 322 and coding the current frame of digital signal samples and coding the transition frame includes transform coding and predictive coding of a single sub frame 321 of the transition frame And comprising the following sub-steps:
- determining (611) the distribution of bits according to any one of the preceding claims;
- predicting (614) the transition subframe (321) for a first number of bits allocated; And
- transform decoding (612) the transition frame (322) for a second number of bits allocated. 15. A computer program comprising instructions for implementing a method according to any one of the preceding claims when instructions are executed by a processor. An apparatus for determining a distribution of bits for coding transition frames (321, 322), the apparatus (104, 503) being implemented in a coder / decoder for coding / decoding a digital signal, Wherein the step of coding the transition frame comprises transform coding and predecoding a single subframe (321), wherein the number of bits for coding the transition frame is fixed The apparatus comprising a processor arranged to perform the following operations,
Allocating a bit rate for predicting the transition subframe, the bit rate being equal to a minimum value between a bit rate for transform coding the transition frame and a first predetermined bit rate value;
Determining a first number of bits allocated for predicting the transition sub-frame for the bit rate;
And - a second number of bits allocated for transcoding the transition frame from the number of bits needed to code the coding parameters and the number of fixed bits for coding the transition frame. A coder capable of coding digital signal frames in accordance with predictive coding or transform coding,
Device (104) according to claim 13; And
A prediction coder 103 comprising a processor configured to perform the following operations,
- code the preceding frame of digital signal samples according to prediction coding;
- coding a current frame of digital signal samples, coding the transition frame, and transcoding or predecoding the subframe, wherein the processor is configured to: The number of transitions being arranged to predictively code subframes;
- a transcoder (105) comprising a processor arranged to perform transcoding the transition frame at a second number of bits allocated. In a decoder for a digital signal coded by predictive coding and transform coding,
An apparatus (503) according to claim 13; And
A prediction decoder (504) comprising a processor configured to perform the following operations,
Predictively decode the preceding frame (320) of coded digital signal samples according to predictive coding;
Predicting and decoding a single subframe (321) included in a transition frame, coding a current frame of digital signal samples, coding the transition frame, and predictively coding and transform coding the subframe, And to perform an operation of predicting and decoding the transition sub-frame of the first number of bits,
And a transform decoder (507) comprising a processor arranged to perform transform decoding of the transition frame for a second number of bits allocated.

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